Download The possible gametic alleles

Part 2: Heredity and
Mendelian Genetics
The Inheritance of Two Traits
Shhhh!
Peas pay attention
Mendel’s Second Experiment:
A Dihybrid Cross
Mendel’s second major experiment
involved the crossing of two pea plants
that differed in TWO traits.
Mendel wanted to discover if the
inheritance of one trait is influenced by
the inheritance of another trait. (i.e. Did
pea colour influence pea shape?)
Once again Mendel crossed two plants
that were pure breeding.
This time he
observed two
different traits.
The Determination of Gametes for a
Dihybrid Cross.
When generating the possible gametes for a
dihybrid cross, one must predict the entire
complement of possible gametes. If the F1
generation are hybrids, then each parent
can each produce four possible gametes.
The possible gametic alleles
Each hybrid F1 individual has the
genotype:
RrYy
The possible gametic alleles are:
RY, Ry, rY, ry
Nine different genotypes and four
different phenotypes result from a
dihybrid cross of F1 plants.
Mendel discovered that all of the F1
generation were hybrid for both traits
(round and yellow).
Mendel then crossed the F1 generation and
discovered that the F2 generation had a
phenotypic ratio of 9:3:3:1. The F2
generations of other dihybrid crosses also
showed this ratio.
Of the 551 plants in Mendel’s F2
generation, he observed the following
traits:
320 round yellow
101 wrinkled yellow
104 round green
26 wrinkled green
Mendel’s Law of Independent
Assortment
The inheritance of alleles for one trait does
not influence the inheritance of alleles for
another trait.
This means that the combination of alleles in
the offspring may not occur in either parent.
(i.e. they sort independently.)
Probability
Genotypic and phenotypic ratios are determined by the
probability of inheriting a certain trait. The probability of an
event is the likelihood that the event will occur. Probability
can be expressed by the following formula:
In humans, free earlobes are controlled by
the dominant allele E, and attached
earlobes by the recessive allele e. The
widow’s peak hairline is regulated by the
dominant allele H, while the straight hairline
is controlled by the recessive allele h
Sample Problem 1
What are the probabilities of obtaining F1 offspring with the
following characteristics if one parents is homozygous
dominant for both traits and the other is heterozygous dominant
for both ?
• widow’s peak and free earlobes
• straight hairline and free earlobes
• widow’s peak and attached earlobes
• straight hairline and attached earlobes
Check your answers by completing a Punnet Square!
Sample Problem 2
What is the probability that a child from the mating of
the EeHh × EeHh parents
would be a male with a widow’s peak and have
attached earlobes? Write your probability as a
percentage!
Mono and Dihybrid Cross Assignment Coming Soon!
Beyond Mendel’s Laws
Although his research and examination of
patterns of inheritance in the pea plant
was revolutionary in genetics; Mendel did
not have a complete understanding of the
patterns of inheritance.
There are three other mechanisms of
inheritance we will discuss in this
course.
Incomplete Dominance
Co-dominance
Multiple Alleles
Incomplete Dominance
Not all traits are completely dominant or
recessive as Mendel suggested.
Some traits are incompletely dominant,
that is, there can be an intermediate
expression of a particular trait when the
genotype is heterozygous.
The snapdragon flower is incompletely
dominant for flower colour.
Two red alleles are required for a red flowe
i.e. RR
Two white alleles are required for a white
flower i.e. R’R’
A heterozygous set of alleles is required fo
a pink flower i.e. RR’
The R’ allele is called “R prime”
Incomplete Dominance in the
Snapdragon Flower
Sample 1.
Determine the F1 phenotypic ratio of a cross
between a pink and a white snapdragon.
Co-dominant Inheritance
In some cases both alleles for a trait may
be dominant.
Such alleles are said to be co- dominant
because both alleles are expressed in a
heterozygous individual.
Shorthorn cattle have the mechanism of
co-dominant inheritance for their coat
colour.
The expression of both alleles occurs and there is
no blending of traits.
The homozygous red coat alleles are: CRCR (called
“C superscript R)
The homozygous white coat alleles are: CWCW
The heterozygous roan coat alleles are: CRCW
Sample 1.
Find the F1 phenotypes of a cross between a red cow and a roan bull.
Multiple Allelic Inheritance
For some traits more than two alleles
control the expression in a species
Although a single individual cannot have
more than two alleles for each trait,
different individuals can have different pairs
of alleles.
Human blood types have multiple
allele inheritance.
Sex Linkage - X and Y
Some traits are inherited from genes on the sex
chromosomes
This is known as sex-linked (most often X-linked)
inheritance.
Colour blindness, hemophilia, and baldness are sex-linked
traits.
A male with hemophilia mates with a woman with no
hemophiliac gene. What is the probability of producing sons
or daughters who have hemophilia?
Pedigree Charts
Pedigree analysis is useful when the traits of many
generations of offspring have been recorded.
A pedigree chart can be used to trace the passing of an
allele from parents to offspring.
A pedigree chart contains a number of symbols.
1. Indicate whether each family member is homozygous or heterozygous for
shortsightedness, or homozygous for normal vision.
2. If couple 4 and 5 in row II had another child, what genotype might the child
have? (Hint: What genotype is possible but not shown in the chart?) Would the
child have normal vision or be shortsighted?